Independent screwed wellheads are well known in the art and classified by the American Petroleum Institute (API). The independent screwed wellhead has independently secured heads for each tubular string supported in the well bore. Independent screwed wellheads are widely used for production from low-pressure productions zones because they are economical to construct and maintain.
It is well known in the art that low pressure wells frequently require some form of stimulation to improve or sustain production. Traditionally, such stimulation procedures have involved pumping high pressure fluids down the casing to fracture production zones. The high pressure fluids are often laden with proppants, such as bauxite and/or sharp sand.
FIG. 1 illustrates a prior art independent screwed wellhead 20 equipped with a flanged casing pin adaptor 30 typically used for completing or re-completing a well equipped with an independent screwed wellhead 20. The independent screwed wellhead 20 is mounted to a surface casing (not shown). As is understood by those skilled in the art, the independent screwed wellhead 20 may have a screw connection, a welded connection, a clamped connection or a pin connection to the surface casing. The independent screwed wellhead 20 includes a sidewall 32 that terminates at a top end in a casing bowl 34, which receives a casing mandrel 36. The casing mandrel 36 has a bottom end 38, a top end 40 and an axial passage 42 having a diameter at least as large as a casing 44 in the well bore. The casing 44 has a pin thread 46 that engages a box thread 48 in the bottom end 38 of the casing mandrel 36. A flanged casing pin adaptor 30 has a pin thread 47 that engages a box thread 49 in the top end of the axial passage 42 in the casing mandrel 36. The flanged casing pin adaptor 30 also includes a top flange 45 to which a high pressure valve or a blowout preventor (BOP) is mounted, in a manner well known in the art.
In a typical well stimulation procedure, a casing saver (not shown), such as a casing packer as described in U.S. Pat. No. 4,993,488, which issued Feb. 19, 1991 to Macleod, is inserted through the BOP (not shown) and into the casing 44. The casing saver is sealed off against the casing 44 and high pressure fluids are injected through the casing saver into a formation of the well. While the casing saver protects the exposed top end of the casing 44 from “washout”, it does not release the box thread 49 or the pin thread 47 from strain induced by the elevated fluid pressures generated by the injection of high pressure fracturing fluid into the well. In a typical fracturing operation, high pressure fluids are pumped into the well at around 9500 lbs per square inch (PSI). If “energized fluids” or high pumping rates at more than 50 barrels per minute are used, peak pressures can exceed 9500 PSI. In general, the threads retaining the flanged casing pin adaptor 30 in the casing mandrel 36 are engineered to withstand 7000 PSI, or less. Consequently, high pressure stimulation using the equipment shown in FIG. 1 can expose the flanged casing pin adaptor 30 to an upward pressure that exceeds the strength of the pin thread. If either the box thread 49 or the pin thread 47 fails, the flanged casing pin adaptor 30 and any connected equipment may be ejected from the well and hydrocarbons may be released to atmosphere. This is an undesirable and potentially dangerous situation.
Furthermore, use of a casing saver to perform well completion or re-completion slows down operations in a multi-zone well because the flow rates are hampered by the reduced internal diameter of the casing saver. Besides, the casing saver must be removed from the well each time the fracturing of a zone is completed to permit isolation plugs or packers to be set in order to isolate a next zone to be stimulated. It is well known in the art that the disconnection of fracturing lines and the removal of a casing saver is a time consuming operation that keeps expensive fracturing equipment and/or wireline equipment and crews sitting idle. It is therefore desirable to provide full-bore access to the well casing 44 in order to ensure that transitions between zones in a multi-stage fracturing process are accomplished as quickly as possible.
Applicant has developed an independent screwed wellhead that overcomes these problems by providing an external thread on a top end of a casing mandrel of the wellhead. The external, or “pin”, thread is used for lock down connection of well stimulation components. The external thread permits a double-locking of a well stimulation tool to the wellhead, as is described in detail in co-pending, U.S. patent application Ser. No. 10/440,795 entitled CASING MANDREL WITH WELL STIMULATION TOOL AND TUBING HEAD SPOOL FOR USE WITH THE CASING MANDREL, which was filed on May 19, 2003, and is incorporated herein by reference. The well stimulation tool and casing mandrel provide full-bore access to a casing of the well, while significantly improving safety of a well stimulation crew by ensuring that a hold strength of equipment through which well stimulation fluids are pumped exceeds fluid injection pressures by an adequate margin.
While the casing mandrel of the co-pending application represents a significant advance in the art, there remains a need for lowering a height of the casing mandrel and the well stimulation tool, and for improving versatility of the combination.